Secondary batteries, which are exemplary electrochemical devices, have widely proliferated not only as power sources of portable devices such as mobile phones, digital cameras and laptop computers but also as vehicle or household power sources. In particular, a lightweight lithium ion secondary battery with a high energy density is an energy accumulation device that is indispensable for daily life. Among the secondary batteries, a laminated type secondary battery is configured having an electric storage element in which a plurality of positive and negative electrode sheets are alternately laminated multiple times with separators interposed therebetween and the electric storage element is contained in an outer container with an electrolyte.
In such a secondary battery, a shock, a vibration or the like that is received from a source that is outside of the outer container can produce a positional shift of the electric storage element in the interior of the outer container, and thereby, can cause a decrease in reliability of electric connection, a decrease in energy density, or can damage the flexible film as a result of an edge portion of the electric storage element coming into contact with the inner surface of the flexible film.
Hence, Patent Document 1 discloses a battery configured such that a part of the separator protrudes laterally from the positive electrode and the negative electrode, a through-hole is provided on the protrusion portion, and an upper flexible film and a lower flexible film that make up the outer container come into direct contact with each other and are welded in the through-hole. Patent Document 2 discloses a battery configured such that a part of the separator protrudes laterally from the positive electrode and the negative electrode, and the protrusion portion comes into contact with and is welded to upper and lower flexible films.
In recent years, the increase in the capacity of the battery and the increase in the energy density of the battery have increased the risk of significant heat generation, and therefore, enhancement of heat resistance has been desired. In particular, if the separator that is positioned between the positive electrode and the negative electrode is thermally fused, or if the separator that is positioned between the outermost electrode (typically, the negative electrode) and the flexible film that makes up the outer container is thermally fused, thermal fusion can produce an electric short circuit and can cause an additional and greater increase in heat generation, and therefore, heat resistance properties are desired. For example, Patent Document 3 discloses a separator composed of a fiber assembly whose melting point is 150° C. or higher, and Patent Document 4 discloses a separator composed of a porous membrane whose glass transition temperature is 70° C. or higher.